MAKEDEV is the preferred way of creating
device files which are not present. However sometimes the
MAKEDEV script will not know about the device
file you wish to create. This is where the mknod
command comes in. In order to use mknod you need
to know the major and minor node numbers for the device you wish to
create. The devices.txt file in the kernel
source documentation is the canonical source of this
information.

To take an example, let us suppose that our version of the
MAKEDEV script does not know how to create the
/dev/ttyS0 device file. We need to use
mknod to create it. We know from looking at the
devices.txt file that it should be a character
device with major number 4 and minor number 64. So we now know all
we need to create the file.

As you can see, many more steps are required to create the file. In
this example you can see the process required however. It is
unlikely in the extreme that the ttyS0 file would not be provided by
the MAKEDEV script, but it suffices to illustrate
the point.